1. Field of the Invention
The present invention relates to an apparatus for forming a deposited film which forms, on a substrate, a deposited film functional and useful for various devices, e.g., semiconductor device, image-inputting line sensor, photographing device, and electrophotographic photosensitive member, more particularly an apparatus for continuously forming a semiconductor device composed of a thin film of large area, e.g., photoelectromotive device, on a long, band-shaped substrate.
2. Related Background Art
One of the apparatuses for forming a deposited film, which continuously forms thin films on a substrate, is based on continuous plasma CVD method using a roll-to-roll method, as disclosed by U.S. Pat. No. 4,400,409 specification. This apparatus includes two or more glow discharge regions, in which sufficiently long, band-shaped substrates of given width are arranged in a route along which they pass the glow discharge regions orderly, and continuously transfers these substrates in the longitudinal direction while depositing a required electroconductive semiconductor layer in each glow discharge region, to continuously form a large-area device with semiconductor joints. It is considered that these characteristics make the roll-to-roll method suitable for mass production of large-area semiconductor devices.
One of the major problems the above apparatus should solve is controlling deposition of the film for each semiconductor layer to have a uniform thickness, when the semiconductor-joined devices having functional deposited films of different composition, e.g., solar cells, are to be massively produced, because the band-shaped substrates passing over each semiconductor layer is transferred at a constant speed. The CVD method for such an apparatus widely uses RF discharge as means for exciting a glow discharge plasma for decomposing the material gas and forming the deposited film, which tends to cause significant dispersion of film thickness and quality, resulting from increased flow rates of material gas, increased power input, and also increased area of parallel plate electrode. These problems will greatly aggravate the above-described problems involved in the apparatus working based on continuous plasma CVD method using the roll-to-roll method, because they tend to adversely affect film thickness and quality when the band-shaped substrate transferring speed is changed, and also form undesirable by-products, e.g., polysilane, which may deteriorate the deposited film.
The apparatus which uses the roll-to-roll method for forming the deposited film generally includes a number of discharge vessels, to secure desired thickness of the deposited film by continuously transferring the band-shaped substrate in the longitudinal direction.
The apparatus for forming the deposited film, based on continuous plasma CVD method using the conventional roll-to-roll method, is considered to be suitable for mass production of the semiconductor devices of large area.
However, in terms of mass production of the apparatus for forming the deposited film which adopts the plasma CVD method using the roll-to-roll method, it is essential to increase rate at which the band-shaped substrate is transferred. For example, for the mass production, it is necessary to sufficiently increase number of the discharge vessels to secure a desired thickness for each semiconductor layer, in order to increase the transferring rate, because rate of film deposition in each discharge vessel is limited. This tends to increase size, complexity and cost of the apparatus.
The improved productivity is a major problem for the above apparatus to solve for the mass production. This inevitably involves increased rate at which the film is deposited, and extension and increased number of the discharge vessels, leading to increased size, complexity and cost of the apparatus. In particular, the increased size of the discharge space causes the various problems, described below.
The deposited film produced in each discharge vessel is affected by various conditions, e.g., type of discharge energy for generating the plasma, discharge conditions, and composition, flow rate, flow velocity (exhaust velocity) and pressure of the material gas in the vacuum vessel. When the deposited film is continuously produced, in particular, dispersion of the flow velocity (exhaust velocity) of the material gas, resulting from by the viscous effect of the side walls in the vacuum vessel, causes its stagnation of the material gas, which, in turn, causes formation of by-products in the deposited film forming and gas exhausting spaces, to possibly deteriorate the deposited film.
Moreover, the by-products formed in the deposited film forming and gas exhausting spaces may be deposited on the parallel plate electrode and exhaust port, further aggravating dispersion of the material gas flow velocity (exhaust velocity), which, in turn, further aggravates dispersion of the deposited film quality and thickness. Dispersion of the deposited film thickness and quality, and deposition of the film on an undesired site on the band-shaped substrate in each discharge vessel are serious problems, particularly for the deposited film forming method in which the film is deposited over a large area on the continuously transferred band-shaped substrate.